Method of and apparatus for digesting wood



June 9, 1931.

J. E. BQND 1,808,830

METHOD OF AND APPARATUS FOR DIGESTING WOOD Fil'ed May 27, 1929 1 \i 22252% ll 8 25 26 I 0 I9 20 I v 5 J8 J7 I l q M 01-0! I Y 215202172257Wcfizess l Patented June 9, 1931 UNITED STATES PATENT OFFICE JOSEPHE.BOND, OF APPLETOIN', WISCONSIN METHOD OF AND APPARATUS FOR DIGESTIN'GWOOD Application filed May 27, 1929. Serial No. 366,295.

paratus are commonly employed. This process admittedly has manyshortcomings in the quantity of cellulose produced and in the efficiencyof the process as a whole but is used in preference'to other moreeflicient methods because of the relatively quicker timewithin whichpulp may be produced. The process of digestion consists in subjectingthe chips to the action of an acid,- and depends for its success on thedifference in solubility of the important constituents of wood: that is,the lignin, sugar and resin of wood are relatively more soluble in theparticular acid employed than cellulose. However, cellulose underextreme conditions of strength of acid, temperature, pressure andtime issoluble in the same acid and, therefore, the process must be carried onin such manner that the more soluble constituents may be attacked anddissolved without impairing the strength and hydration capacity of thecellulose.

Inasmuch as the wood must for practical purposes be treated in the formof chips or other divisions of substantially uniform size the difiicultyencountered is that of securing penetration of the acid to the interiorparts of thersmall body of wood before the exterior portions have beentreated to such an extent that the uality of the cellulose fibers isharined. xperience has shown that during the first hour or two in thedigester, the temperature should be 110 C. in order to prevent theformation of an insoluble lignin compound around the chips which rendersthe chips impervious to subsequent treatment. However, it is desirableto raise the temperature of the mass within the digester as quickly aspossible. This involves the admittance of steam at high pressure andusually there is a percentage ofburned or useless chips that havebeenrendered useless by proximity to the steam inlet. 50 It is a wellknown physical fact also that solubility isgreatly increased where thereis eflicient agitation; this for the reason that if the solvent and thebody to be dissolved remain quiescent the solvent immediately around thebody becomes saturated and its efliciency is lost. In the processesheretofore employed, circulation of the chips has been effected solelyby means of steam jets, although in some cases the acid without thechips has been circulated, as in the Mitscherlich process.

My invention contemplates, in effect, a' combination of the advantagesof both the RittenKellner and the Mitsch'erlich processes, and, inaddition, a radical departure from the practice commonly employed. Forexample, I accomplish the quick saturation and cooking of the directprocess with the ad vantages of the indirect process.

The invention will be more readily understood by reference to theaccompanymg drawings, and will be'more specifically pointed out in thesubsequent description of the apparatus employed. i In the accompanyingdrawings, Fig. l is a vertical sectional view through a digesterconstructed in accordance with my invention, the illustration beingsomewhat diagrammatic as'to certain details; and Fig. 2 'is a sectionalview on the line 22' of F 1.

' The apparatus which I prefer, to employ consists of a lined shell 10having a covered inlet opening 11 at the top. through-which the chipsare admitted. The casing has a conical bottom 12 and is supported'in theusual manner.

' As a means for carrying out the process of my invention, I make adeparture from the usual -digester construction by providing a largecentrally disposed pipe 13 having an open top that terminates short ofthe upper end of the digester. The lower end is in communication withthe discharge outlet of a centrifugal or screw pump 14, the impeller ofwhich is indicated at 15. The im p-eller is mounted on a relatively longshaft 116 having outboard bearings 1718 and actuated by a motor 19. Theinlet portion 20 of the pump casing is extended to receive a pluralityof ship propellers 21 mounted on the shaft 16, having the function ofdisintegrating the chips, breaking up any lumps or clumpsof material andgenerally assisting in the agitation. The inlet pipe 22 is in Icommunication with the lower end of the conical portion of the shell.

Arranged exteriorly of the digester in a,

suitable casing 23 is an acid heating tank containing steam coils 24. A-pipe 25 connects with the inlet pipe 22 and a centrifugal pump 26' inthe pipe line 25 withdraws acid from the digester, passes it through thetank and returns it through the pipe 27 to the inlet casing 22 adjacentto the zone of operation of the propellers 21. Fresh acid may beadmitted through the pipe 25a, as may be necessary at the start ofoperations.

Also connected to the casing 21 is a pipe 28 adapted to admit steam ifand when required.

Notwithstanding that the digester is filledwith chips at the startofoperation, the bulk will be reduced as the operation proceeds. I havetherefore extended the pipe 13 near to the top of the casing to insurethe flow of acid over the top of the chips. -However, as

digestion proceeds, the bulk will be lessened and there is no necessityfor raising the material materially above the level in the digester. Ihave,therefore,provided means such as the sleeve 29 within the pipe foropening tube 13. The chips in the tube beingloosely and closing ports 30when it is unnecessary.

to raise the material to the top of the pipe. The sleeve may be operatedby means of a screw 31 having an exterior handwheel and. connected to ashaft'32 rigid with the sleeve.

The operation of the apparatus in carrying out my improved process is asfollows:

The digester and the pipe 13 are filled with chips in the usualmanner.After the filling opening is closed, acid is pumped through the pipe 25aand 27 and caused to rise in the circulation of both the chips and theacid will begin. If the chips are dry, steam'will be admitted throughthe pipe 28 along with the acid. The rapid movement of the chips andacid and the complete agitation atthe .point' of admission of'the steamwill prevent burning, as the steam will be condensed very quickly.

In order to quickly raise the temperature I of the mass within thedigester without diluting the acid with an excessive amount of steam, Iwithdraw acid through the pipe 25,

and pass it through the heater returning it to the pump inlet inproperly heated condi- "tion.

By indirectly heating the acid and by thorough agitation of the digestercontents,

the time of cooking is materially reduced and the yield of cellulose isgreatly increased.

The cost of operation is also materially reexpense in. the recovery ofS0 By eflt'ectivev circulation and the accurate control of temperatureduring the penetration or satura. tion period, I am able to reduce theper centage of burned chips, slivers, and tailings and the mechanicaldevice employed for circulation assists materially in the disintegrationof the wood chips.

The apparatus employed is typical only and other forms may be devisedand utilized to advantage without departure from the spirit of myinvention.

I claim: v 1

1. The method of operating a digester having a vertical open top tubetherein which consists in charging chips into the digester and tube thendelivering hot acid to the bottom of the tube and causing it to rise andoverflow over the top of the chips in the space around the tube, thencontinuously pumping acid and chips from the bottom of the space aroundthe tube and discharging said acid and chips upwardly through said tube,withdrawing cooled acid from the digester, indirectly heating the sameat a point exterior of the digester and returning the heated acid to themoving body of material.

2.. The method of operating a digester having a vertical open top tubetherein which consists in charging chips into the digester and tube thendelivering hot acid to the bottom of the tube and causing it to rise andoverflow over the top of the chips in the space around the tube, thencontinuously pumping acid and chips from-the bottom of the space aroundthe tube and discharging said acid and chips upwardly through said tube,Withdrawing "cooled acid from the bottom of the digester and supplyinghot acid to the body of material that is being discharged into saidtube.

' 3. The method of operating a digester which consists in supplyingchips thereto,

then supplying hot acid and steam while con-- tinuously and mechanicallycirculating the the contents. of the digester until the temperature ofthe contents is raised to approximately C., then continuing thecirculation ofthe contents and maintaining the temperature substantiallyconstant until saturation is complete, then raising the temper? ature toa proper coking temperature and continuing the circulation of the chipsand acid. I

4. The combination of a digester having .the inlet to said pump, andmeans in the intake for disintegrating the chips.

5. The combination of a digester having a vertical open top tubetherein, a pump having its inlet connected to the bottom of the spacesurrounding the tube and its outlet connected to "said tube, means forsupplying hot acid to the digester at a point adjacent to the inlet tosaid pump, and a ship propeller in the intake for disintegrating saidchips before they enter the pump. v

6. The combination of a digester having a vertical open top tubetherein, a pump having its inlet connected to the bottom of the spacesurrounding the tube and its outlet connected to said tube, and meansfor withdrawing cooled acid from the bottom of the digester, heating theacid exteriorly of the digester and returning the heated acid to thepump.

7. The combination of a digester having a. vertical open top tubetherein, a pump having its inlet connected to the bottom of the spacesurrounding the tube and its outlet connected to said tube, means forcontinuously. withdrawing cooled acid from the lower portion of thedigester, means for indirectly heating the withdrawn acid, and means-forreturning the heated acid to the moving body of chips and .acid passingthrough the pump.

8. Ina digester the combination with the casing of a vertical open toppipe therein, a pump having its outlet connected to the pipe and itsinlet connected to the bottom of the casing, and means for varying theeffective length of the pipe.

9. In a digester the combination with the casing of a vertical open toppipe therein, a pump having its outlet connected to the pipe and itsinlet connected to the bottom of the casing, means providing a port inthe pipe at a point below its upper end, andmeans operable from theexterior of the digester for opening said port when the level ofmaterial in the digester is adjacent to said port.

In testimony whereof I have aflixed my signature.

' JOSEPH E. BOND.

